Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/156

Rh were the oldest of the Echinodermata, the Brachiopoda of Mollusca, the Entomostraca of Crustacea, and that the Worms preceded the Insects, etc. We have just seen that the life of the Silurian, the most ancient period except the Azoic, was characterized by these very orders, which are the most simply organized of the respective divisions of the animal kingdom, while the Fucoidae, or brown sea-weed, found fossil in rocks of this period, belong to the Algae, the simplest division of the vegetal kingdom. Geological evidence confirms, therefore, not only in a general way, but to an extent in detail not to be hoped for from the nature of the subject, the view of the development of the animal and vegetal kingdoms deduced from their structure. The Silurian period is sometimes called the Age of Mollusca and Algae.

Passing from the Silurian period to the Devonian, so called from the rocks of this formation having been first studied in Devonshire, England, we notice that while the first half of the Devonian agrees in its main features with the latter half of the Silurian, the latter half of the Devonian, often called the Old Red Sandstone, offers evidence of a progress in life, since its rocks contain the remains of fish, together with a few Ferns, Lycopods, and Conifers. The remains of these plants are, however, only rarely found in the Devonian; the flora of this period, as well as that of the Silurian, being more generally characterized by the presence of Algae. The Fishes found in the Devonian period are Sharks and Ganoids (Fig, 147). The Sharks belong to the order of Cestraphori, or weapon-bearers, so named from their dorsal fin being armed with a long spine; these spines